Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

Chao Peng
2022-11-01T11:49:08Z

On Mon, Oct 31, 2022 at 12:47:38PM -0500, Michael Roth wrote:
> On Tue, Oct 25, 2022 at 11:13:37PM +0800, Chao Peng wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> >
> > Introduce 'memfd_restricted' system call with the ability to create
> > memory areas that are restricted from userspace access through ordinary
> > MMU operations (e.g. read/write/mmap). The memory content is expected to
> > be used through a new in-kernel interface by a third kernel module.
> >
> > memfd_restricted() is useful for scenarios where a file descriptor(fd)
> > can be used as an interface into mm but want to restrict userspace's
> > ability on the fd. Initially it is designed to provide protections for
> > KVM encrypted guest memory.
> >
> > Normally KVM uses memfd memory via mmapping the memfd into KVM userspace
> > (e.g. QEMU) and then using the mmaped virtual address to setup the
> > mapping in the KVM secondary page table (e.g. EPT). With confidential
> > computing technologies like Intel TDX, the memfd memory may be encrypted
> > with special key for special software domain (e.g. KVM guest) and is not
> > expected to be directly accessed by userspace. Precisely, userspace
> > access to such encrypted memory may lead to host crash so should be
> > prevented.
> >
> > memfd_restricted() provides semantics required for KVM guest encrypted
> > memory support that a fd created with memfd_restricted() is going to be
> > used as the source of guest memory in confidential computing environment
> > and KVM can directly interact with core-mm without the need to expose
> > the memoy content into KVM userspace.
> >
> > KVM userspace is still in charge of the lifecycle of the fd. It should
> > pass the created fd to KVM. KVM uses the new restrictedmem_get_page() to
> > obtain the physical memory page and then uses it to populate the KVM
> > secondary page table entries.
> >
> > The userspace restricted memfd can be fallocate-ed or hole-punched
> > from userspace. When these operations happen, KVM can get notified
> > through restrictedmem_notifier, it then gets chance to remove any
> > mapped entries of the range in the secondary page tables.
> >
> > memfd_restricted() itself is implemented as a shim layer on top of real
> > memory file systems (currently tmpfs). Pages in restrictedmem are marked
> > as unmovable and unevictable, this is required for current confidential
> > usage. But in future this might be changed.
> >
> > By default memfd_restricted() prevents userspace read, write and mmap.
> > By defining new bit in the 'flags', it can be extended to support other
> > restricted semantics in the future.
> >
> > The system call is currently wired up for x86 arch.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> > arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> > arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> > include/linux/restrictedmem.h | 62 ++++++
> > include/linux/syscalls.h | 1 +
> > include/uapi/asm-generic/unistd.h | 5 +-
> > include/uapi/linux/magic.h | 1 +
> > kernel/sys_ni.c | 3 +
> > mm/Kconfig | 4 +
> > mm/Makefile | 1 +
> > mm/restrictedmem.c | 250 +++++++++++++++++++++++++
> > 10 files changed, 328 insertions(+), 1 deletion(-)
> > create mode 100644 include/linux/restrictedmem.h
> > create mode 100644 mm/restrictedmem.c
> >
> > diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> > index 320480a8db4f..dc70ba90247e 100644
> > --- a/arch/x86/entry/syscalls/syscall_32.tbl
> > +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> > @@ -455,3 +455,4 @@
> > 448 i386 process_mrelease sys_process_mrelease
> > 449 i386 futex_waitv sys_futex_waitv
> > 450 i386 set_mempolicy_home_node sys_set_mempolicy_home_node
> > +451 i386 memfd_restricted sys_memfd_restricted
> > diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> > index c84d12608cd2..06516abc8318 100644
> > --- a/arch/x86/entry/syscalls/syscall_64.tbl
> > +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> > @@ -372,6 +372,7 @@
> > 448 common process_mrelease sys_process_mrelease
> > 449 common futex_waitv sys_futex_waitv
> > 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> > +451 common memfd_restricted sys_memfd_restricted
> >
> > #
> > # Due to a historical design error, certain syscalls are numbered differently
> > diff --git a/include/linux/restrictedmem.h b/include/linux/restrictedmem.h
> > new file mode 100644
> > index 000000000000..9c37c3ea3180
> > --- /dev/null
> > +++ b/include/linux/restrictedmem.h
> > @@ -0,0 +1,62 @@
> > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> > +#ifndef _LINUX_RESTRICTEDMEM_H
> > +
> > +#include <linux/file.h>
> > +#include <linux/magic.h>
> > +#include <linux/pfn_t.h>
> > +
> > +struct restrictedmem_notifier;
> > +
> > +struct restrictedmem_notifier_ops {
> > + void (*invalidate_start)(struct restrictedmem_notifier *notifier,
> > + pgoff_t start, pgoff_t end);
> > + void (*invalidate_end)(struct restrictedmem_notifier *notifier,
> > + pgoff_t start, pgoff_t end);
> > +};
> > +
> > +struct restrictedmem_notifier {
> > + struct list_head list;
> > + const struct restrictedmem_notifier_ops *ops;
> > +};
> > +
> > +#ifdef CONFIG_RESTRICTEDMEM
> > +
> > +void restrictedmem_register_notifier(struct file *file,
> > + struct restrictedmem_notifier *notifier);
> > +void restrictedmem_unregister_notifier(struct file *file,
> > + struct restrictedmem_notifier *notifier);
> > +
> > +int restrictedmem_get_page(struct file *file, pgoff_t offset,
> > + struct page **pagep, int *order);
> > +
> > +static inline bool file_is_restrictedmem(struct file *file)
> > +{
> > + return file->f_inode->i_sb->s_magic == RESTRICTEDMEM_MAGIC;
> > +}
> > +
> > +#else
> > +
> > +static inline void restrictedmem_register_notifier(struct file *file,
> > + struct restrictedmem_notifier *notifier)
> > +{
> > +}
> > +
> > +static inline void restrictedmem_unregister_notifier(struct file *file,
> > + struct restrictedmem_notifier *notifier)
> > +{
> > +}
> > +
> > +static inline int restrictedmem_get_page(struct file *file, pgoff_t offset,
> > + struct page **pagep, int *order)
> > +{
> > + return -1;
> > +}
> > +
> > +static inline bool file_is_restrictedmem(struct file *file)
> > +{
> > + return false;
> > +}
> > +
> > +#endif /* CONFIG_RESTRICTEDMEM */
> > +
> > +#endif /* _LINUX_RESTRICTEDMEM_H */
> > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> > index a34b0f9a9972..f9e9e0c820c5 100644
> > --- a/include/linux/syscalls.h
> > +++ b/include/linux/syscalls.h
> > @@ -1056,6 +1056,7 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
> > asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
> > unsigned long home_node,
> > unsigned long flags);
> > +asmlinkage long sys_memfd_restricted(unsigned int flags);
> >
> > /*
> > * Architecture-specific system calls
> > diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> > index 45fa180cc56a..e93cd35e46d0 100644
> > --- a/include/uapi/asm-generic/unistd.h
> > +++ b/include/uapi/asm-generic/unistd.h
> > @@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
> > #define __NR_set_mempolicy_home_node 450
> > __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
> >
> > +#define __NR_memfd_restricted 451
> > +__SYSCALL(__NR_memfd_restricted, sys_memfd_restricted)
> > +
> > #undef __NR_syscalls
> > -#define __NR_syscalls 451
> > +#define __NR_syscalls 452
> >
> > /*
> > * 32 bit systems traditionally used different
> > diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> > index 6325d1d0e90f..8aa38324b90a 100644
> > --- a/include/uapi/linux/magic.h
> > +++ b/include/uapi/linux/magic.h
> > @@ -101,5 +101,6 @@
> > #define DMA_BUF_MAGIC 0x444d4142 /* "DMAB" */
> > #define DEVMEM_MAGIC 0x454d444d /* "DMEM" */
> > #define SECRETMEM_MAGIC 0x5345434d /* "SECM" */
> > +#define RESTRICTEDMEM_MAGIC 0x5245534d /* "RESM" */
> >
> > #endif /* __LINUX_MAGIC_H__ */
> > diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> > index 860b2dcf3ac4..7c4a32cbd2e7 100644
> > --- a/kernel/sys_ni.c
> > +++ b/kernel/sys_ni.c
> > @@ -360,6 +360,9 @@ COND_SYSCALL(pkey_free);
> > /* memfd_secret */
> > COND_SYSCALL(memfd_secret);
> >
> > +/* memfd_restricted */
> > +COND_SYSCALL(memfd_restricted);
> > +
> > /*
> > * Architecture specific weak syscall entries.
> > */
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 0331f1461f81..0177d53676c7 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -1076,6 +1076,10 @@ config IO_MAPPING
> > config SECRETMEM
> > def_bool ARCH_HAS_SET_DIRECT_MAP && !EMBEDDED
> >
> > +config RESTRICTEDMEM
> > + bool
> > + depends on TMPFS
> > +
> > config ANON_VMA_NAME
> > bool "Anonymous VMA name support"
> > depends on PROC_FS && ADVISE_SYSCALLS && MMU
> > diff --git a/mm/Makefile b/mm/Makefile
> > index 9a564f836403..6cb6403ffd40 100644
> > --- a/mm/Makefile
> > +++ b/mm/Makefile
> > @@ -117,6 +117,7 @@ obj-$(CONFIG_PAGE_EXTENSION) += page_ext.o
> > obj-$(CONFIG_PAGE_TABLE_CHECK) += page_table_check.o
> > obj-$(CONFIG_CMA_DEBUGFS) += cma_debug.o
> > obj-$(CONFIG_SECRETMEM) += secretmem.o
> > +obj-$(CONFIG_RESTRICTEDMEM) += restrictedmem.o
> > obj-$(CONFIG_CMA_SYSFS) += cma_sysfs.o
> > obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
> > obj-$(CONFIG_IDLE_PAGE_TRACKING) += page_idle.o
> > diff --git a/mm/restrictedmem.c b/mm/restrictedmem.c
> > new file mode 100644
> > index 000000000000..e5bf8907e0f8
> > --- /dev/null
> > +++ b/mm/restrictedmem.c
> > @@ -0,0 +1,250 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#include "linux/sbitmap.h"
> > +#include <linux/pagemap.h>
> > +#include <linux/pseudo_fs.h>
> > +#include <linux/shmem_fs.h>
> > +#include <linux/syscalls.h>
> > +#include <uapi/linux/falloc.h>
> > +#include <uapi/linux/magic.h>
> > +#include <linux/restrictedmem.h>
> > +
> > +struct restrictedmem_data {
> > + struct mutex lock;
> > + struct file *memfd;
> > + struct list_head notifiers;
> > +};
> > +
> > +static void restrictedmem_notifier_invalidate(struct restrictedmem_data *data,
> > + pgoff_t start, pgoff_t end, bool notify_start)
> > +{
> > + struct restrictedmem_notifier *notifier;
> > +
> > + mutex_lock(&data->lock);
> > + list_for_each_entry(notifier, &data->notifiers, list) {
> > + if (notify_start)
> > + notifier->ops->invalidate_start(notifier, start, end);
> > + else
> > + notifier->ops->invalidate_end(notifier, start, end);
> > + }
> > + mutex_unlock(&data->lock);
> > +}
> > +
> > +static int restrictedmem_release(struct inode *inode, struct file *file)
> > +{
> > + struct restrictedmem_data *data = inode->i_mapping->private_data;
> > +
> > + fput(data->memfd);
> > + kfree(data);
> > + return 0;
> > +}
> > +
> > +static long restrictedmem_fallocate(struct file *file, int mode,
> > + loff_t offset, loff_t len)
> > +{
> > + struct restrictedmem_data *data = file->f_mapping->private_data;
> > + struct file *memfd = data->memfd;
> > + int ret;
> > +
> > + if (mode & FALLOC_FL_PUNCH_HOLE) {
> > + if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
> > + return -EINVAL;
> > + }
> > +
> > + restrictedmem_notifier_invalidate(data, offset, offset + len, true);
> > + ret = memfd->f_op->fallocate(memfd, mode, offset, len);
> > + restrictedmem_notifier_invalidate(data, offset, offset + len, false);
> > + return ret;
> > +}
>
> In v8 there was some discussion about potentially passing the page/folio
> and order as part of the invalidation callback, I ended up needing
> something similar for SEV-SNP, and think it might make sense for other
> platforms. This main reasoning is:

In that context what we talked on is the inaccessible_get_pfn(), I was
not aware there is need for invalidation callback as well.

>
> 1) restoring kernel directmap:
>
> Currently SNP (and I believe TDX) need to either split or remove kernel
> direct mappings for restricted PFNs, since there is no guarantee that
> other PFNs within a 2MB range won't be used for non-restricted
> (which will cause an RMP #PF in the case of SNP since the 2MB
> mapping overlaps with guest-owned pages)

Has the splitting and restoring been a well-discussed direction? I'm
just curious whether there is other options to solve this issue.

>
> Previously we were able to restore 2MB mappings to some degree
> since both shared/restricted pages were all pinned, so anything
> backed by a THP (or hugetlb page once that is implemented) at guest
> teardown could be restored as 2MB direct mapping.
>
> Invalidation seems like the most logical time to have this happen,

Currently invalidation only happens at user-initiated fallocate(). It
does not cover the VM teardown case where the restoring might also be
expected to be handled.

> but whether or not to restore as 2MB requires the order to be 2MB
> or larger, and for GPA range being invalidated to cover the entire
> 2MB (otherwise it means the page was potentially split and some
> subpages free back to host already, in which case it can't be
> restored as 2MB).
>
> 2) Potentially less invalidations:
>
> If we pass the entire folio or compound_page as part of
> invalidation, we only needed to issue 1 invalidation per folio.

I'm not sure I agree, the current invalidation covers the whole range
that passed from userspace and the invalidation is invoked only once for
each usrspace fallocate().

>
> 3) Potentially useful for hugetlbfs support:
>
> One issue with hugetlbfs is that we don't support splitting the
> hugepage in such cases, which was a big obstacle prior to UPM. Now
> however, we may have the option of doing "lazy" invalidations where
> fallocate(PUNCH_HOLE, ...) won't free a shmem-allocate page unless
> all the subpages within the 2M range are either hole-punched, or the
> guest is shut down, so in that way we never have to split it. Sean
> was pondering something similar in another thread:
>
> https://lore.kernel.org/linux-mm/YyGLXXkFCmxBfu5U@google.com/
>
> Issuing invalidations with folio-granularity ties in fairly well
> with this sort of approach if we end up going that route.

There is semantics difference between the current one and the proposed
one: The invalidation range is exactly what userspace passed down to the
kernel (being fallocated) while the proposed one will be subset of that
(if userspace-provided addr/size is not aligned to power of two), I'm
not quite confident this difference has no side effect.

>
> I need to rework things for v9, and we'll probably want to use struct
> folio instead of struct page now, but as a proof-of-concept of sorts this
> is what I'd added on top of v8 of your patchset to implement 1) and 2):
>
> https://github.com/mdroth/linux/commit/127e5ea477c7bd5e4107fd44a04b9dc9e9b1af8b
>
> Does an approach like this seem reasonable? Should be work this into the
> base restricted memslot support?

If the above mentioned semantics difference is not a problem, I don't
have strong objection on this.

Sean, since you have much better understanding on this, what is your
take on this?

Chao
>
> Thanks,
>
> Mike